While carrying the amulet, casting spells takes more power than usual. The penalty is between 1 and 2n power points, where n is the normal power cost of the spell cast. This means spell costs will be doubled on average, and may use up to three times as much power in the worst case.[12]

Effects of the mysterious force

If you try to go upstairs in Gehennom while carrying the Amulet of Yendor, the mysterious force may instead teleport you down (increase your dungeon level) by 0 to 3 levels. A level change of zero teleports you somewhere else in the level (teleport control does not apply). The chance depends on your alignment.

Level change

Lawful

Neutral

Chaotic

​−1

75.00%

75.00%

75.00%

0

6.25%

8.33%

12.50%

+1

11.45%

12.50%

12.50%

+2

5.20%

4.17%

0

+3

2.08%

0

0

Avg. Change

-0.470

-0.542

-0.625

Effects while wearing/wielding the Amulet of Yendor

When worn or wielded, you may get messages about the amulet getting "hotter" as you approach magic portals.

Every game turn there is a 1/15 chance that you will receive a message if you are near a magic portal.[13]

Distance

Message

3 square radius

"The Amulet of Yendor feels hot!"

8 square radius

"The Amulet of Yendor feels very warm."

12 square radius

"The Amulet of Yendor feels warm."

Farther

No message

Offering the amulet

If you offer the amulet on the correct high altar on the Astral Plane, you ascend and win the game. Offering the amulet on another god's high altar will also end the game: your current god is angered, but the altar's god allows you to escape in celestial disgrace. This is effectively an ascension, but is less prestigious because it removes the need to find the correct altar.

Attempting to offer the amulet on any other altar gives the message “You feel an urge to return to the surface.”, or if hallucinating, “You feel homesick.” There is no penalty for this, even if you attempt to do it at a cross-aligned or unaligned altar.

Offering to Moloch

Moloch shrugs and retains dominion over <your god>, then mercilessly snuffs out your life.
Do you want your possessions identified?

The game ends in death, with the cause listed as “Moloch's indifference”. If you survive, by wearing an amulet of life saving, the message continues:

Moloch snarls and tries again...

Then, Moloch disintegrates you into a pile of dust even if you are disintegration resistant; the cause of death is “the wrath of Moloch”.

Should you somehow survive this, the following message will appear:

You are surrounded by a cloud of black soot...
Do you want your possessions identified?

This will be treated as escaping the dungeon.

Messages

You feel the amulet draining your energy away.

You cast a spell while carrying the amulet. Between 1 and 2n extra Power points were used, where n equals to the Power originally needed to cast the spell. This Power drain is incurred before the normal Power used for the spell is calculated, resulting in spells occasionally being unable to be cast due to a lack of energy. Ideally, even with 0% spell failure, you will require 3n Power to ensure a spell always works.

Beware of forgeries!

It is possible to find a Cheap plastic imitation of the Amulet of Yendor at several places in the Mazes of Menace. These generally appear to the player as "The Amulet of Yendor," but messing up and offering one of them to your god carries a penalty to luck, and possibly alignment and your god's anger (if you attempt to offer a known fake). But the real danger lies in becoming confused by the abundance of fakes and carrying one off instead of the real amulet if, for instance, the Wizard steals it from you. There are several ways to avoid this fate. Many players will immediately #[name] the real Amulet "REAL" or something similar immediately upon claiming it from the High Priest. Another technique is to attempt to put the Amulet into a container; the real Amulet will resist, but a fake will go in. Also, quaffing a cursed potion of gain level on dungeon level 1 will make you "have an uneasy feeling" if you don't have the real Amulet, but take you to the Plane of Earth if you do.

UnNetHack

In UnNetHack the amulet has a chance of randomly teleporting within the level when dropped for any reason. This is in many ways balanced by the removal of the mysterious force. If cursed, the chance of the amulet teleporting is high, but if blessed, the likelihood is significantly reduced. Cursed amulets teleport 1/2 of the time, uncursed 1/4 of the time, and blessed 1/16 of the time.

Sacrificing the Amulet of Yendor breaks atheist conduct in UnNetHack, other than in vanilla nethack. The Player will be warned if he attempts to sacrifice the amulet. In order to complete the game, one must invoke the amulet on the corresponding altar instead.

Encyclopedia entry

This mysterious talisman is the object of your quest. It is
said to possess powers which mere mortals can scarcely
comprehend, let alone utilize. The gods will grant the gift of
immortality to the adventurer who can deliver it from the
depths of Moloch's Sanctum and offer it on the appropriate high
altar on the Astral Plane.

Trivia

The Amulet Of Yendor is named after the Rogue Item, "The Amulet of Yendor", in which it also ends the game!